1. The Field of the Invention
The invention relates to a network communication system and, more specifically, to local area and wide area networks incorporating an ITU/TDD/VOICE modem.
2. The Background Art
The “plain old telephone system” (POTS) has been in development since the late 1800's. Another term for POTS is “public switched telephone network” (PSTN). In general, the term PSTN will mean any medium of communication in which at least part of the link is analog, and not digital, in nature. The PSTN was first developed in the late 1800's to allow for person to person communications by voice over long distances. The invention of computers made it useful to exchange digital data over the PSTN. In the 1950's, “modems” were developed for that purpose. A modem accepts digital data, modulates it into a form suitable for transmission over the PSTN such that a corresponding receiving modem can demodulate the signal back into the original digital data.
Computer networks have become very popular in allowing computers to communicate and transmit data across local area networks (LAN) and wide area networks (WAN). The Internet itself could be considered to be a WAN. A LAN or WAN link is digital in nature and conforms to a known protocol such as NETBIOS, TCP/IP, SPX/IPX, etc. over a communication media such as Ethernet, Token Ring, or higher speed media.
Several modem protocols of the format “V dot” protocol include the V.32, V.34, and V.70 protocol. All such “V dot” protocols are promulgated by the International Telecommunications Union (ITU), the governing body for modem standards based in Geneva Switzerland. The earlier “Bell” standards, such as Bell 103, Bell 212, etc., are similar but earlier modem standards adopted in the United States prior to the U.S.'s joining of the ITU. The full specifications of any ITU standard may be obtained from the ITU.
The well known “2500” series analog phones will connect directly to the PSTN but not directly to a LAN or WAN. In the PSTN, often the link from a business or residence to the “CO” (central office) is analog, although much of the rest of the PSTN may be digital. Modems such as V.32, V.34, etc. are designed to operate on an analog or mixed analog/digital link. The term PSTN in the discussion below will sometimes include a private branch exchange (PBX) or key phone systems or links as found in many organizations. Examples of such PBX or key systems include AT&T's “Merlin”—or “Partner Plus” systems, Nortel's SL1, SL100, or Northstar systems, and so on. PBX or key systems will often connect directly to analog lines from the PSTN, although digital connections, such as T1 lines, are common. A voice caller on a PBX or key system is able to call and converse with an analog 2500 series phone whereas such a phone will not connect directly to a LAN or WAN network.
Modems provide a means for the deaf or speech impaired to communicate over the telephone system, since typed characters can be sent over the PSTN via modem and read by another modem user. However, standard modems have evolved with poor provision for communications by the deaf or speech impaired and are largely unsuitable for this purpose. Standard modems were developed primarily for computer to computer communications. For example, most standard modems have a “carrier” which is a constant audible signal. If this signal is lost because the call is placed on hold, a standard modem will generally hang up and cannot reestablish the connection with a different modem. Modems are very inflexible in their protocol requirements and will not operate properly or will abort the call if the protocol settings are wrong.
In the late 1960's, three deaf engineers developed a new type of a modem called a TDD or “telecommunication device for the deaf”, sometimes called a TTY. A TDD is typically a stand alone modem system with an integral keyboard and display that allows typed characters to be exchanged with another TDD over the PSTN. Because of its simple protocol requirements, the TDD is robust and easy to use. Consequently, the TDD is, in many respects, more suitable than are standard modems for communications by the deaf.
The International Telecommunications Union (“ITU”) (formerly known as the CCITT) has been active in setting various modulation standards for modems. Examples of ITU standards are V.21, V.22, V.22 Bis, V.32, V.32 Bis, and V.34. In the United States, prior to the adoption of ITU standards, various “Bell standards” were established. Examples of Bell standards are Bell 103 and Bell 212. As used herein, an “ITU modem” is a modem that may operate using an ITU modulation/demodulation protocol (or standard). Typically, ITU modems may also operate in at least one Bell modulation/demodulation protocol. A “Bell modem” may operate in a Bell modulation/demodulation protocol. Most modems currently being sold in the United States are ITU modems that are capable of transmitting and receiving in various ITU modulation/demodulation protocols as well as Bell modulation/demodulation protocols. The term “ASCII modem” is sometimes used to refer to an ITU or Bell modem.
A standard modem is a modem that runs all or some of the modem standards passed by the ITU (International Telecommunications Union), or the earlier “Bell” protocols such as Bell 103, Bell 212A, etc. Modem standard modems will typically support protocols such as Bell 103, V.34 (28,800 bps), V.32bis (14,400 bps), V.22bis (2400 bps), and so on. As used here, it is assumed that a “standard modem” does not support one or more of the modem protocols used by the deaf, such as 45 or 50 bps Baudot, EDT (European Deaf Telephone), the DTMF text telephones used in Europe, or etc. As used herein, a “standard modem” is a modem other than a TDD that follows an established protocol such as an ITU or Bell modulation/demodulation protocol and transmits and/or receives data over a communication medium. The communication medium may be a telephone system (such as the PSTN), PBX, or other media such as microwave link, coax, or fiber optic cable. ITU modems and Bell modems are examples of standard modems.
Differences between standard modems and TDDs include: (1) TDDs encode text characters with the generally obsolete five bit Baudot code, while most modems and desktop computers in the United States today use an ASCII encoding, also known as the ANSI X3.4-1977 or ASCII-77 encoding, (2) TDDs do not provide for parity bits, (3) TDDs in the United States transmit/receive only at 45.45 bits per second, and (4) TDDs use frequency shift keying modulation/demodulation at frequencies of 1400 and 1800 Hz. Because TDDs do not use ITU modulation and protocol schemes, TDDs and ITU modems are incompatible and cannot communicate with each other.
Dual purpose modems have been developed that can communicate with both TDDs and ITU modems. Some of these dual purpose modems have the capability to determine whether incoming and outgoing signals are in TDD format or standard modem format.
The deaf or speech impaired find many products, methods, or techniques used in modem communications to be difficult or impossible to use. These problems are made even more evident with the integration of networks into communication technology. The descriptions contained here will show how new techniques and technologies can provide improved communications for the deaf and/or speech impaired.